This invention relates to conductive composite compositions of the conducting form of polyaniline and benzobisazole polymers and co-polymers.
In the field of conducting polymers, great strides have been made in the development of polyaniline (PANI)-based materials for a broad spectrum of applications ranging from electronic and optoelectronic devices to anti-corrosion coatings to gas separation membranes. PANI exists in primarily four oxidation states: the completely reduced form, leucoemeraldine base (LEB): ##STR1## the intermediate form, emeraldine base (EB): ##STR2## the fully oxidized form, pernigraniline base (PNB): ##STR3## and the protonated state of the intermediate form, emeraldine salt (ES): ##STR4## where A is a counterion, e.g., Cl.sup.-, HSO.sub.4.sup.-, etc. Only the emeraldine salt (ES) is conducting. The ES form can be converted from LEB by oxidation and protonation, from PNB by reduction and protonation and from EB by simply protonation; these processes are chemically and electrochemically reversible.
Because of its good solubility in aprotic solvents such as N-methylpyrrolidinone (NMP), PANI as the emeraldine base (EB) is commonly used to prepare films. However, in this state, PANI is an insulator and a doping step is thus required in the fabrication process. Upon doping with protonic acid, the resultant emaraldine salt (ES) becomes conducting with typical conductivity values of 10.sup.0 to 10.sup.2 S/cm, depending on synthesis and processing conditions.
A major breakthrough was the discovery that the use of organic sulfonic acids, such as camphor sulfonic acid (CSA) and p-dodecylbenzenesulfonic acid (DBSA), as the protonic dopants not only provided high conductivity, but also provided solubility of PANI in its conducting state (emeraldine salt, ES) in organic solvents such as m-cresol, chloroform, hexafluoroisopropanol, and the like. This has provided a means for preparing conducting blends with controllable conductivity and thermo-mechanical properties from conducting PANI-ES and a wide range of thermoplastics and elastomers.
Unfortunately, this simple blending using non-acidic, organic solvents is not applicable to aromatic heterocyclic rigid-rod polymers such as poly(benzobisazoles) (PBX), whose solubility is limited to strong acids such as methanesulfonic acid (MSA) and poly(phosphoric acid) (PPA) and Lewis acid/nitroalkanes. These polymers have repeating units of the general formula --(--Z--Ar--)--, wherein Z is a benzobisazole group and Ar is a para-oriented aromatic moiety, such as 1,4-phenylene, 4,4'-biphenylene, 4,4'-diphenylsulfide, 4,4'-diphenylether or the like. Thus, the group includes poly(p-phenylene benzobisoxazole) (PBO), poly(p-phenylene benzibisthiazole) (PBT) and poly(p-phenylene benzobisimidazole) (PBI) polymers and co-polymers, as well as substituted derivatives thereof. The simpler PBX polymers have repeating units of the formula ##STR5## wherein X is --O--, --S--or --NH--.
The preparation and processing of rigid-rod polymers and co-polymers is well known in the art. They are commonly prepared by the condensation of at least one di- or tetra-amino monomer with at least one para-oriented dicarboxylic acid monomer in a strong acid such as polyphosphoric acid (PPA). The polymer or co-polymer is commonly recovered from the reaction mixture by precipitation in water. In this context, the term `coagulation` is often used interchangeably with `precipitation`. The polymer or co-polymer can be formed into desirable shapes by directly extruding the PPA mixture into water. Alternatively, the polymer or co-polymer can be coagulated or precipitated into water, washed, dried and then taken up in a strong acid, such as sulfuric acid or methanesulfonic acid (MSA), then extruded, cast or spin-cast into desirable shapes followed by coagulation in water.
Rigid-rod polymers and co-polymers generally have very high thermal stability, high mechanical strength and low solubility in ordinary solvents. These polymers and co-polymers generally have melting temperatures higher than their thermal degradation temperatures; thus, they cannot be processed by melt-processing techniques. As noted previously, they are normally processed into desirable shapes by extrusion, casting or spin-casting from a strong acid solution, such as from a methanesulfonic acid or polyphosphoric acid solution.
Accordingly, it is an object of the present invention to provide new conductive composite compositions comprising the conducting form of polyaniline and aromatic benzobisthiazole, benzobisoxazole and benzobisimidazole (PBX) rigid-rod polymer.
It is another object to provide a process for making such conducting films.
Other objects and advantages of the present invention will be apparent to those skilled in the art.